This invention relates to an extension unit for a door lock, and more particularly to an extension unit that can be added to a conventional latch unit to adjust the backset of the latch unit.
Generally speaking, a door lock comprises a latch tube having a latch bolt and a guard bolt, with the latch tube being connected to a latch actuating unit. Such a door lock typically has two standard backsets, with backset being defined as the distance from the door edge to the axis of the knob. Standard backsets are usually either 23/8 inches (60 mm), or 23/4 inches (70 mm).
In order to supply a latch unit that will accommodate both standard backsets, prior art designs had to supply, on the same production line, a respective latch unit comprising the same components, but differing only in backset length. Consequently, prior art latch units resulted in inefficient production, high cost, and inconvenience in manufacture, sales and distribution.
Numerous prior art means have been used to make a latch unit assembly in which the backset can be transformed by interposing various types of extensions, or changing the structure of the latch tube to provide an increased backset between the rear of the latch tube having a standard backset and the latch bolt actuating unit.
An example of such prior art is shown in U.S. Pat. No. 3,046,042, issued July 24, 1962, to J. F. Kane. While this is designed to increase the backset of a conventional latch bolt unit by securing an extension unit consisting of an extension link, a latch bar extension, and a cylindrical housing, it has some disadvantages.
The extension unit, because of its complicated components, must be handled many times in manufacturing. Also, the unit employs direct reciprocating movement of the latch bar extension utilizing only a pair of finger elements which may result in reducing the life, or limiting the operational efficiency of the extension unit.
Another prior art patent, U.S. Pat. No. 4,496,178, issued Jan. 29, 1985 to W. E. Best et al, discloses an extension tube which is riveted to a conventional latch tube in order to provide an increased backset. However, said extension tube, which comprises a cross bar, a connector, etc., is of complicated construction and is integrally riveted to the latch tube. Therefore, transformation to a latch unit with an extended backset or replacement of the latch tube with another backset cannot be easily accomplished. As a result, the manufacturing process for making this latch tube with different backsets requires separate manufacturing steps, thereby lowering production efficiency.
Still another prior art patent is U.S. Pat. No. 4,653,787, issued Mar. 31, 1987, to Y. P. Fang. This patent discloses a backset-adjustable latch of a cylindrical lock which can transform a latch with a backset of 23/8 inches (60 mm) to a backset measuring 23/4 inches (70 mm).
Backset is adjusted simply by pressing down a case fixing button located on the extension case, whereby the extension case is extended to provide an increased backset.
The Fang patent discloses specific components, e.g., a latch bolt extension slide, a guard bolt extension slide, and a retractor slide, which components, in combination, provide a latch unit with increased backset. From a manufacturing point of view, while this patent has a convenient extension mechanism, it contains many complicated components in order to provide adjustable backset and, thus, results in a waste of time and effort in the manufacturing procedures.
It is, therefore, an object of the present invention to provide an extension unit for a door lock which can transform a latch unit with a backset of 23/8 inches, to a backset of 23/4 inches, and advantageously be easily removed or secured to the endwall of the latch tube.
It is a still further object of the present invention to provide an extension unit which is simply constructed for efficient manufacturing and for ease of handling.